Shelving the shingles: the costs and benefits of green roofs

A worker helped set up the green roof at the Woodbury police station. (Photo: Bill Klotz)

So-called green roofs have been hailed as one of the most effective tools to reduce the negative environmental effects of conventional buildings.

They can also reduce the expense of cooling and heating a building, add two decades or more to the lifespan of a roof membrane, and are considered useful in controlling storm water runoff — another cost saver.

But they cost more to install, in some cases twice as much per square foot as a conventional roof. And they add weight that might require building reinforcement and some maintenance.

For a building owner, does installing a green roof makes sense from a bottom-line perspective? Those who sell the roofs, and building design and construction experts, say there is no simple answer.

Performance data to calculate the cost-benefit ratio of specific green roofs is hard to come by. For example, the Minneapolis Central Library has three green roofs totaling 18,000 square feet on its building, which opened about five years ago. The green roofs are part of a storm water control system that produces significant savings in the library’s utility bills.

But there’s no way to exactly calculate energy savings, because officials do not know how much energy the new library would have consumed if it had a conventional roof, said Nathan Leif, a senior property manager for Hennepin County.

When considering whether to choose a green roof, building owners should consider not only the bottom-line elements such as installation and maintenance costs but also the societal benefits, said Scott Nelson, chief executive officer of St. Paul-based VWI Sustainability Consulting. “People may install a green roof to save money or because it’s the right thing to do,” or both, he said.

To reduce the amount of work needed to maintain a green roof, choosing the correct type of plants is essential, Nelson pointed out. Grass and other indigenous plant species — chosen for drought tolerance — are preferred to cut the need to water the roof.

Another factor: Because green roofs with plants and soil add significant weight, buildings that use them must be significantly reinforced. More often than not, it’s not practical or cost-effective to retrofit existing buildings for that purpose. But working a green roof in the design of a new building is more likely to be cost-effective, Nelson said.

The Minnesota Green Roofs Council‘s database lists 159 green roofs installed so far in the state. Almost all are atop public buildings such as college and university facilities, hospitals, government buildings and multifamily residential developments. Few have been installed by for-profit businesses.

Minneapolis developer Michael Lander has used green roof systems at ground level to cover the tops of underground parking garages on two of his condo projects, which “is a little different from an actual green roof,” he said.

The added green space at ground level, in place of a hard surface, is a landscaping amenity for condo residents, he says.

Lander is a booster of the green-roof concept, but he points out that there are more immediate, cost-effective ways to reduce building energy use — including insulation and well-sealed windows and doors.

Why haven’t more commercial businesses in Minnesota installed green roofs yet? “Green roofs are great, and we’re going to see more of them in the future,” Lander said. “But until the industry adopts a warranty and has more of a track record, a lot of business owners wonder if they have proven” their effectiveness and durability.

The relative lack of construction activity since the Great Recession has also reduced private-sector green roof activity.

Installation costs are a major factor, said Stacy Anderson, president of Earth Wizards, a Minneapolis-based green paving and landscaping company.

“Sometimes a property owner thinks how cool it would be to have a green roof, but when you start talking about the price it gets tossed out, and we end up installing a storm water control system or permeable pavement (to absorb runoff) instead,” Anderson said.

Calculating a “payback period” can only be done on a project-by-project basis.

Much of the current information on roof longevity is based on computer-modeled projections, said Angie Durhman, green roof manager for roofing contractor Tecta America Corp. “But at the same time,” she said, “people are doing hard-core scientific research to show the true benefits of green roofs. So we’re starting to get some better information.”

Generally, installers’ and manufacturers’ warranties on green roofs range between 10 and 20 years, Durhman said. But any warranty is “only as good as the green roof’s design,” she said.

According to a study in Germany, a “vegetated roof” on average can be expected to prolong the service life of a conventional roof by at least 20 years.

When the long-term savings from deferred maintenance and energy conservation are factored in, vegetated rooftops are comparable in cost to conventional roofs, research indicates.

Another consideration is whether a green roof will be visible. People typically think of landscaping as an amenity and don’t consider the cost-benefit analysis, said D’Andre Willis, an associate vice president at Minneapolis-based HGA architects.

“If a roof is visible or accessible, it can have enormous benefit as an amenity,” Willis said. “In a hotel or hospital, a green roof can make a huge difference in how you feel about being there.”

As more green roofs are installed, manufacturers continue to improve the product.

Minneapolis-based Bachman’s sells LiveRoof, a pre-seeded, modular green roof system made by a Michigan company. The “pre-vegetated” system establishes rooftop plants more quickly with significantly less watering required, said Doug Danielson, a sales representative for Bachman’s.

One version of the system features soil that is 2.5 inches deep (rather than the usual 4 inches), reducing the weight load on a roof to about 15 to 17 pounds per square foot (when fully saturated) compared with about 29 pounds per square foot or more with most green roof systems.

The LiveRoof costs $15 to $20 per square foot compared with an average of about $8 for a conventional roof, Danielson said.

One comment

What one chooses to use as the green makes a difference. In the Portland area, sedums are frequently used. One that will grow here is purslane, which is beautiful, can be edible, and is probably going to die back and dry up in the winter when severe cold hits.

There are tall sedums and there are prostrate ones. Because they sequester moisture when it is there and give it up when there is not, there are possibly other advantages to using these.

Other plants that require no inputs but that can be harvested might also be good. Alliums are pretty cold-hardy, many of them, and the roots are shallow. Some, like shallots, actually have been known to harvest themselves, that is to push themselves up out of the substrate when they want you to take them in.

One thing I find nice about alliums is that you can eat the bulb and replant the roots. The flowers are also pretty. Many garlics make these round flowers at the end of sturdy stems. I tell the neighborhood kids they are magic wands.